On Tuesday, Independence Day eve, Joel Pomerantzthought the gang that would gather at the corner of McAllister and Divisadero for his Divisadero Water History walk might be small. Wind might come up, it might be chilly, it was the evening before a holiday.

Nonetheless, standing in front of a photo backdrop he'd created in the former doorway of a Divisadero health food shop, at 7:30 p.m., the creator of Thinkwalks (thinkwalks.org) is ready to go. It turns out that 20 or so have shown up, and Pomerantz begins by having each of us introduce ourselves. Most are San Franciscans, and many live in the neighborhood - a six-block or so area - the tour will cover.

Pomerantz, a tall man with an easy but not glib manner, is used to leading tours around the non-touristy parts of the city: Among this summer's Thinkwalks offerings are "The Wiggle Neighborhood Natural & Cultural History" and an Outside Lands bike ride through Golden Gate Park. He carries a cloth bag containing copies of various topographical maps, laminated so they can be handed out and passed around among tour-goers.

He's not interested in little anecdotal tidbits, he warns attendees. Pomerantz is a big-picture guy. "Knowing how the world really fits together, or at least considering the question deeply, is essential in making your own impact," says his literature. In person, he admits that his personal passion is groundwater.

Around the corner from our meeting spot, Pomerantz tells us the neighborhood was all sand as late as the second half of the 19th century, and that we're standing on the site of an ancient trail that went through the dunes 5,000 years ago. The trail connected two villages of the Yelamu (Ohlone) people: Chutchui, near what became Mission Dolores, and Petlenuc, near the Presidio.

At Alamo Square, Pomerantz dips his hand into lawn made spongy by underground springs, and speaks of many other natural things, including the ridge of serpentine rock on which the park sits, which runs on a diagonal across the city from Hunters Point to the southern pier of the Golden Gate Bridge.

Some two hours after it had begun, the tour is winding up, at the corner of Broderick and Hayes. We're standing between two sites of former bodies of water: San Souci Lake at Divisadero and Grove was the location of a roadhouse, one of the first buildings in the area; and Phelps Lake - formed, he thinks, in the winter of 1862 during heavy rainfall that became known as the Great Flood of California - which extended three-quarters of a mile from east to west, with its southern shore around the Abner Phelps house on Oak Street. (The house has been moved, but still sits on Oak, on a rise behind a lawn.)

Pomerantz says he found a March 1862 newspaper account that described a rainstorm so heavy that Phelps Lake flooded through the sand berms around it and rushed downhill all the way to Church and Market, where it destroyed the country house of French-born financier and developer Francois Pioche, one of the pioneers of San Francisco.

Standing on the street corner, Pomerantz is in the middle of this enthusiastically told tale when suddenly, he is hailed by a passing driver, someone he apparently knows well. After ascertaining that Pomerantz is doing one of his tours, the driver pulls his car over, leaps out and runs over to our group.

"I'm going to Seventh and Clarendon," he says. "Who wants to go to the sewers?" After propounding the wonders of that particular spot, where a lake runs next to the roadway, he repeats his invitation, adding, "I've got an extra pair of rubber boots in the car." Sure enough, one of the Thinkwalk tour-goers whispers something to his partner, leaves him and jumps in the car to take off for a field trip within our field trip, to the Inner Sunset. Pomerantz tells us the driver-host is Greg Braswellof the San Francisco Department of Public Works Bureau of Engineering; he knows his way around sewers.

Then out of that cloth bag comes a plastic jar, which is passed around for donations, a sliding scale, from $10 to $40. Some folks repair to Chili Pie on Baker Street for an after-snack. And that's how it was in the hood on Tuesday night.